1. Field of the Invention:
This invention relates to power supply circuits and specifically to series switching regulator power supplies utilizing transistors operated in parallel.
2. Description of the Prior Art:
It is the characteristic of switching transistors that the switching response varies significantly from transistor to transistor. This characteristic exhibits itself in the form of a slightly varying saturation voltage, i.e., the voltage across the transistor when it is conducting and has adequate base drive to maintain a minimum voltage drop across the transistor. The difference between transistors also is exhibited in the form of frequency response which shows up as differing slopes on the leading edge and trailing edge of a pulse when the transistor is turned on and off with a fast rectangular waveform. The transistor also exhibits different storage times. The storage time is the duration of conduction after the base drive is removed.
In the art of developing high reliability power supplies, it is known to have a power supply in which a DC power is generated by conventional means and the power supply is voltage or current regulated by switching the load on and off at a high rate by using switching transistors. It is also known to filter the DC output between the switch mechanism and the load. It is characteristic of high voltage, high speed, switching transistors that they be limited to collector currents of the order of 5 to 20 amperes and are generally limited to about 5 amperes for efficient, reliable, high voltage regulation.
A common technique is to parallel more than one transistor to increase the current and reduce the load on each single transistor. When this is done, the transistors exhibit differing switching characteristics and a single transistor usually ends up carrying an inordinate share of the current during some period of the switching cycle. To reduce that problem, emitter resistors and emitter transformers have been employed. These devices attempt to sense the emitter current and alter the distribution of base drive between the transistors in an attempt to balance the emitter currents. However, since a switching regulator operates the transistor as a switch, the transistor is not necessarily capable of being driven by the base in such a fashion that balance is accomplished. This can be seen by noting that in a group of transistors, if one transistor is slower than its companion transistors, no amount of base drive will necessarily make it as fast. Therefore, after all the fast transistors have ceased conducting in a turn off cycle, the slow transistor may continue to conduct, and subsequently fail.